1. Field of the Invention
The present invention relates to an optical transmission system for simultaneously transmitting transfer data and its clock signal, or transfer data and its control signal, and more specifically to a parallel optical transmission system connected via a plurality of optical cables between boards and between devices in an information processing system and information transmission switch system.
2. Description of the Related Art
Recently in an information communications network, there are increasing requests for the transfer of image data in addition to voice and data communications.
In response to these requests, various services are provided at different transmission speeds. When files are transferred in a LAN, etc., data may be transferred in data bursts. ATM systems have become popular as an effective system for practically realizing such services using a single communications system.
In an ATM system, transmission data are divided into 48-byte fixed-length payloads. A 5-byte header indicating routing information, etc. is added to each payload to form a cell. Thus, data are transferred at an optional speed by controlling the number of cells to be transmitted.
In the ATM system, cell data are processed in parallel to enhance the efficiency of a switching process. At this time, the cell data are transmitted in parallel between devices forming a switch system. Since an ATM system is normally large in scale, the devices are connected to each other via metal cables such as coaxial cables, twisted pair lines, etc. provided in parallel. The cell data are transmitted via the cables.
Recently, for services in which data such as video-on-demand data, that is, animation reproducing data, etc, are transferred, the data should be exchanged and transferred at a higher speed. Therefore, in a switch system where data are concentrated, a data transfer speed of several gigabits/second is required between the devices. With the increasing size of the switch system, the distance between the devices becomes longer. Thus, it is required to transfer data at a high speed between the devices distant to each other in the switch system.
Based on the above described background, an optical fiber has been applied to a parallel transmission line for connection between devices in a switch system. At this time, each of the devices in the switch system is connected in parallel via a plurality of optical fibers. For example, the cell data are transferred in parallel in 8-bit or 16-bit units in the switch system. A clock signal and control information about the cell data are transferred simultaneously with the cell data.
Also in a transmission system other than the ATM system, data to be transferred are transmitted in parallel through a plurality of optical fibers, and a control signal is also transferred simultaneously with the data.
The prior technologies relating to the parallel optical transmission are, for example, Tokukaihei 5-183526, Tokukaihei 5-227243, Tokukaihei 6-120743, Tokukaihei 5-29688, etc.
If a fault occurs in the above described optical transmission line, the fault is fundamentally detected on the receiving side. That is, a device on the receiving side detects the presence or absence of significant data by determining an arriving cell to be significant or insignificant (idle cells). If no cells arrive at the receiving side from the transmitting side (for example, at the switching module from the subscriber interface device), the device detects that the level of the optical signal received by the device on the receiving side indicates a value smaller than a predetermined value. Thus, the device determines a disconnection, etc. Otherwise, the physical form of the optical connector for connecting the optical transmission apparatus to the optical fiber is designed such that the connection/disconnection of the optical connector allows failure to be successfully detected.
However, the conventional systems are not provided with the capabilities of controlling the optical output when the above described failure occurs. Therefore, if no cells arrive at the receiving side from the transmitting side, the optical output from the device on the transmitting side is not stopped, but the data containing significant data are continuously output. Thus, the data output from the device on the transmitting side are lost before arriving at the device on the receiving side. Especially, with the increasing data transfer speed in transmitting data in parallel transmission lines, the amount of lost data also increases. Since the device on the transmitting side continues outputting optical signals, a light is output from a connector of the optical fiber in the device on the receiving side when the connector is removed in the device on the receiving side. This is not desired in consideration of safety if the light level is high.
When the system is restored from the state in which no signals arrive from the transmission side to the normal state, an effective method of reactivating the system is earnestly demanded, but has not been successfully developed yet.
The above listed problems are not only caused in the ATM system, but also caused in normal optical transmission systems.